Growing dendrites

rh <rh at nospam.com> writes:
> Bill Skaggs wrote:
> > Are you asking about the functional
> > significance of changing spines, or the biochemical processes behind
> > it?
> >
> > -- Bill
>>> Biochemical, please!
>> Basically, why do they get bigger because they're being used often?
> What, chemically or otherwise, has happened in a dendrite to make it
> grow? Has it received a seperate chemical signal to do so, or is it
> incidental to the first communication pasing through?
>> Thanks
The basic signal is a massive calcium influx into the spine, through a
special kind of voltage-dependent membrane channel called the NMDA
receptor, which is a receptor for the neurotransmitter glutamate. The
NMDA receptor only permits calcium influx if two things happen
simultaneously: 1) glutamate is bound to the receptor; 2) the membrane
of the spine is electrically depolarized. The necks of dendritic
spines are narrow enough to restrict calcium flow, so the
concentration reached inside the spine is considerably higher than the
concentration in the neighboring dendrite.
The influx of calcium triggers a cascade of biochemical processes
whose workings are still only poorly understood, though quite a bit of
research has been done on it. Ultimately, the changes in spine shape
may result from changes in actin, which is known to be present in
spines in substantial quantities.
-- Bill